=(4a^2-12ab+9b^2)-2(6a^2-4ab-9ab+6b^2)+(9a^2-12ab+4b^2)
=4a^2-12ab+9b^2-12a^2+26ab-12b^2+9a^2-12ab+4b^2
=a^2+2ab+b^2
=(a+b)^2